Study On Polarity-Reversed Space Charge Characteristics Of XLPE/SR Under Temperature Gradient

Compared with the cable body insulation of single layer dielectric,the composite insulation in the high voltage direct current(HVDC)cable accessory is more likely to accumulate space charge at the interface due to the discontinuity of the dielectric and the complex interface structure.In the line commutated converter high voltage direct current(LCC-HVDC)transmission system,the voltage polarity needs to be reversal when performing the power flow reversal adjustment,and the HVDC cable accessory will withstand polarity reversal electric field.Although it is temporary,an extremely high transient electric field would be applied locally in the insulation due to the cumulative effect of space charge,which seriously affects the insulation performance.At the same time,the thermal effect generated by the conductor current of the HVDC cable during long-term operation will lead to the formation of a temperature gradient distribution of the inner high and the outer low on the insulation layer.Therefore,under the combined action of temperature gradient and polarity reversal voltage,the dynamic accumulation characteristics of space charge and electric field distortion characteristics within the double layer dielectric composite insulation composed of cable and accessory insulation will become more complex and become an urgent problem to be solved.In this paper,the effects of temperature gradient and polarity reversal voltage on the dynamic accumulation characteristics of space charge and electric field distortion in the double layer dielectric composite insulation composed of the HVDC cable body insulation cross-linked polyethylene(XLPE)and accessory insulation silicone rubber(SR)have been studied by the pulsed electro-acoustic(PEA)method to carry out the experimental measurement and numerical simulation.The research results show that:(1)Under polarity reversal voltage,the change of charge polarity at the XLPE/SR interface lags behind the change of the applied voltage polarity and the charge polarity at the electrode,and the interface charge polarity has a hysteresis effect.(2)The space charge and electric field distribution in the steady state before and after voltage inversion approximately present a "mirror" distribution,and the maximal steady state electric field appears on the XLPE side.Due to the hysteresis effect of interface charge polarity change,the electric field in the SR undergoes severe distortion and the maximal transient electric field occurs when the voltage polarity reversal is completed.(3)The temperature gradient affects the transient electric field distribution during the polarity reversal process by changing the space charge distribution before the voltage inversion.As the temperature gradient increases,the electric field distortion in the SR becomes more severe when the voltage polarity reversal is completed,and the maximal transient electric field distortion rate continues to increase,and the maximal transient electric field distortion rate under high electric field is larger than that of the low electric field.(4)Interface charge at low electric field(5kV/mm)is mainly polarized charge generated by Maxwell-Wagner polarization,which can be simulated and theoretically analyzed using the Maxwell-Wagner polarization model.Under high electric field(20kV/mm),the charge injection effect of the electrode dominates,and the injected charge migrates to the XLPE/SR interface and accumulates,forming the interface charge.Using the bipolar charge transport model can simulate the accumulation process of the injected charge at the interface,and could effectively analyze the accumulation characteristics of the interface charge.